Many products which require sterilization in order to be shelf stable and safe for human consumption, such as food, nutritional, and pharmaceutical products, were originally packaged and terminally sterilized in glass containers. Later, metal cans where used for food and ethical nutritional products in an effort to overcome the problems of glass breakage and excessive distribution and handling weight. Currently, the technology associated with sterilization of products in glass and metal containers is well developed.
Regardless of container style and materials of composition (glass, metal, or polymer), the practice of filling and sealing a product into a container and the process of terminally sterilizing the product after the container is sealed are essentially the same. Most products are filled and sealed into the container so as to substantially reduce headspace air. This minimizes the amount of oxygen in the container which will potentially degrade the nutritional and/or medical potency of the product. In rigid containers this practice generates a vacuum and reduces the pressure exerted by the contents during the sterilization process, especially at peak product temperature. Although vacuums can exist at the sealer in semirigid containers, these may decay with time and many times completely dissipate, especially post sterilization. However, a reduction in headspace air does reduce the pressure exerted by the contents during sterilization, comparable to the case of a rigid container.
Two of the more commonly used methods of reducing headspace air during sealing are a hot fill procedure and steam flushing container headspace during the sealing process. In a hot fill procedure the container is filled with the product and sealed at product temperatures above room temperature, approximately 180.degree. F. When the product is cooled, a vacuum develops due to condensing headspace moisture and contracting headspace gases. In the steam flushing process, steam is used to purge the headspace air out of the filled container, and the container is sealed before the steam condenses. As the steam condenses and headspace gases cool, a vacuum develops. Both methods result in a sealed container with substantially reduced headspace air and, in the case of rigid and the more rigid semirigid containers, a vacuum. Thin walled, low panel strength containers designed for hot fill tend to have bottoms which easily deform inward preventing the net external pressure on the container from exceeding the panel strength of the sidewalls and, thus, preventing the sidewalls from paneling. A container's sidewalls panel when its panel strength is exceeded. The panel strength of a container is defined as the net external pressure at which the side walls of an empty, sealed container buckle inward. Thick walled or high panel strength containers tend to be designed with rigid bottoms since thick-walled container panel strengths tend to be high.
Hot fill alone can be used to sterilize the product if it is a high acid product (approximately below pH 4.6). The container is filled with product and the container is sealed at approximately 180.degree. F. The filled container is then rotated end-over-end so that the hot product contacts all surfaces and, finally, it is held hot for approximately five to ten minutes to kill all viable microorganisms. Microorganisms which are viable at low pH are molds and yeasts. If the product is a low acid product, approximately above pH 4.6, the hot fill process does not produce adequate sterility. Terminal sterilization must be used to kill harmful organisms potentially viable above pH 4.6. Terminal sterilization kills potentially viable organisms by raising product and container temperatures to the equivalent of 250.degree. F. for times equivalent to at least 3 minutes, more often, in excess of 10 minutes as determined using established practices to calculate sterilization process time as a function of product temperature history. The time the product and container are held at an elevated temperature can be reduced markedly by using sterilizer and product temperatures in excess of 250.degree. F. Sterilizer and product temperatures well in excess of 250.degree. F. are commonly used in order to reduce sterilization process time and, thus, product degradation while maintaining microbial kill, since product degradation rates tend to be less temperature sensitive than are microbial death rates. Rigid containers designed for these high-temperature, short-time terminal sterilization processes many times must not only be able to endure the filling and sealing processes using either hot fill or steam flushing, but also must be strong enough to withstand positive net internal pressures, often in excess of 20 psi and negative net internal pressures, or vacuums, often less than -10 psi. These pressures are substantially reduced in semirigid containers capable of deforming without exceeding the failure limits of their materials of construction.
More recently, consumers have indicated an increasing preference for plastic containers, due to factors such as: glass container breakage and metal can damage in distribution; glass container manufacturing and distribution costs; safety with respect to potential glass container breakage; product visibility, especially for monitoring nutritional and pharmaceutical product patient intake; and ecological considerations during container manufacture, product distribution, and either container disposal, recycle, or reuse.
Although consumers have indicated a preference for plastic containers, until fairly recently, container and product manufacturers had to adhere to one or more constraints in order to avoid container distortion during terminal sterilization. Container distortion occurs when the container's materials of construction have been taken beyond their failure limits, and there is objectionable, permanent deformation, post sterilization. These constraints include: (a) The use of low-temperature, long-time processes, with sterilizer temperatures of approximately 250.degree. F. or less and process times greater than approximately 60 minutes to heat, hold, and cool the product in the container, this reduces container-to-container product temperature differences and, thus, decreases container-to-container pressure variation throughout the cycle; (b) the maintenance of precise product fill and headspace gas volumes for more uniform container pressures during sterilization; and (c) the use of container sizes and shapes such as cups and bowls which enhance container panel strength. A cup is a container having a ratio of height to major cross-sectional dimension of less than approximately one. For a drawn or thermoformed, cylindrical container this ratio is the ratio of height to the diameter and is called the draw ratio. The relative shortness of a cup gives it high panel strength as compared to containers with draw ratios above one. A bowl is a cup which does not have a majority of its side wall, between the closure or top and the resting surface or bottom, disposed in a vertical orientation. In the case of a cylindrical bowl, a majority of the side wall is not cylindrical but rather is either conical, some other shape, or, possibly, a combination of various shapes. These irregular sidewall shapes increase the panel strength of these type of containers. Plastic cups and bowls tend to have large closures, usually approximately the same size as the major cross-sectional dimension or diameter. Many times flexible closures are used on these types of containers in order to substantially reduce container vacuum, especially during terminal sterilization, so that container panel strength is not exceeded, thus, avoiding container distortion. However, cups, bowls, and containers with flexible closures are not easily sterilized in high-speed, continuous sterilizers, especially those which are reel-style, or agitating types. This potentially impacts product manufactured cost. Also, cups, bowls, and containers with large, flexible closures are not always the most appropriate container for many food, nutritional, and pharmaceutical products.
Steam retorts operating at saturated steam temperatures and pressures traditionally have been used for metal, glass, and high temperature polymeric materials such as polycarbonate. However air must be added to retorts when food is terminally sterilized in plastic containers in order to prevent excessive container deformation when not using high temperature polymers because materials such as polyolefins tend to have little structural strength at retort temperatures. The pressures required to prevent container distortion are a function of product temperatures, product fill, container headspace and headspace gas volume and commonly are determined experimentally, although emperical and theoretical methods also are available, However, when high-speed, high-temperature, short-time terminal sterilization is applied to products in polyolefin and other plastic containers, the container must be designed to deform reversibly during the process in order to compensate for container-to-container internal pressure variability due to product temperature and fill variablities, and return to its approximate original shape. In addition, when high speed, continuous sterilizers are used, the product filled container must be able to deform adequately in order to survive a wide range of internal pressures, due to either rising or falling product temperature, while the product passes through large preheating vessels in the initial portion of the sterilizer and cooling vessels after sterilization. The greater the container's capability to deform without distortion, the larger and fewer are the required preheating and cooling vessels, thus reducing the cost and complexity of the continuous sterilizer. Additionally, if the container is compatible with metal can sterilizers with minor modifications for the addition of air to the cook vessels, change over costs are minimal.
Plastic containers are able to deform in order to provide, minimally, adequate volume increase to compensate for differences in thermal expansion by the product and the container material, dependent on filled container headspace and headspace gas volume. It is preferable that a plastic container have in excess of 15% volume increase and 1% or more volume decrease in order to be used with multiple vessel, high speed sterilizers without container distortion, post sterilization. One proposed solution to this need for a plastic container for high-temperature-short-time, hot fill, and other terminal sterilization processes is a polyolefin container configured like a drawn metal can as disclosed in U.S. Pat. No. 4,880,129. That particular patent proffers as the solution to the problem, the presence of localized thin spots in the container's bottom wall to facilitate volumetric expansion of the container due to inward and outward flexing of the bottom wall during sterilization. The patent discloses that it is critical that the sidewall must be thicker than the bottom wall. Furthermore, the container must be either annealed or preshrunk in order to remove residual stresses and avoid excessive volumetric shrinkage when sterilization temperatures are above 190.degree. F. This increases the cost of these types of containers. It is claimed that the container can be manufactured by either thermoforming or injection blow molding. Both conventional and multilayer injection blow molding processes can be used to form the container. U.S. Pat. No. 4,526,821 proffers a potential multilayer injection blow molding process. However, the need to use containers with thick sidewalls in order to maintain container panel strength, due to excess sidewall thickness variability within individual containers, in combination with the cost of annealing or preshrinking the containers dramatically increases container cost and significantly reduces the financial attactiveness of this prior art container.
It thus apparent that a need exists for an improved plastic container capable of being use din conventional terminal sterilization equipment. It is also apparent that the need exists for an improved plastic container able to survive retort conditions.